observing whether a point is in a triangle???

hi guys. i have a problem. i did others like this but this is different :)

i have three points.let me say (x1,y1), (x2,y2) and (x3,y3).

as you see these points construct a triangle. my base is (x1,y1) point. and we have a fourth point lets say (a,b). a and b are numbers relative to (x1,y1) point. i mean if (a,b)=(3,4) then it is equal to (x1+3,y1+4). and now the problm is: how can i observe that this point is in the triangle. i mean i give some random point and i want to determine whether this point is in the triangle or not.

thanks in advance :)